The present invention pertains to the instrumentation art and, more particularly, to an improved fuel gaging system.
One type of fuel gaging system used extensively in commercial aircraft employs a probe (or a set of probes) in the fuel tank, the capacitance of which changes as a function of the fuel level. Such systems have been based on the assumption that fuel density and its dielectric constant are related in a closely predictable way. FIG. 1 is a plot of fuel density versus dielectric constant and illustrates a straight line plot 10 which forms the basis for one such aircraft fuel gaging system.
The assumed relationship between fuel density and dielectric constant has, in the past, resulted in gaging errors within acceptable limits, usually less than one percent. At the time such gaging systems were designed, fuel was relatively inexpensive and the cost of transporting additional onboard fuel was not thought excessive.
With increasing costs of crude oil, the cost of transporting generous reserves to accommodate a gaging error has become less acceptable. Also, as crude oil is being used from new sources and a wider range of distillation fractions in jet fuels are being employed, the gaging error due to the assumed density/dielectric constant relationship has increased. Referring again to FIG. 1, a relatively recent survey of samples of jet fuel indicates that deviations from the assumed density/dielectric constant graph 10 can be as great as is shown in points X and Y.
In addition, capacitive probe type fuel gaging systems have been susceptible to errors caused by both stray capacitance and capacitance variances as components become wetted with fuel.
A yet further problem with gaging systems used on aircraft is that gaging errors are induced by the aircraft's attitude.
There is a need, therefore, in the capacitive fuel gaging art, particularly for use in commercial aircraft, for means to correct for the above-identified errors.